Class Inheritance

Inheritance lets a derived class reuse and extend a base class with the extends keyword.

What you will learn: the purpose of class inheritance, the rules TypeScript checks, and what its JavaScript behavior looks like in a runnable example.

The derived instance is also an instance of its base at runtime. In a derived constructor, super(...) must initialize the base before you can use this. Use inheritance for true is-a relationships; composition is often simpler when objects merely collaborate.

Key distinction: TypeScript checks the declared types during development; emitted JavaScript carries out the value-level operation at runtime. A type declaration alone never validates untrusted runtime input.

Read this illustration from the first labeled part to the last. Compare it with the example and identify which parts are type checks and which parts run in JavaScript.

This self-contained example uses fixed values, so it runs without reading input or depending on an external service. Paste it into the TypeScript Playground or save it as main.ts.

TypeScript program

class Animal {
  constructor(public name: string) {}
  speak(): string { return `${this.name} makes a sound`; }
}
class Dog extends Animal {
  bark(): string { return `${this.name} barks`; }
}
const dog = new Dog("Pip");
console.log(dog.speak());
console.log(dog.bark());
console.log(dog instanceof Animal);

Verified output

Pip makes a sound
Pip barks
true

Run locally: npx tsc --strict --target es2022 --module commonjs main.ts, then node main.js. For projects with a tsconfig.json, build using that configuration instead.

Dog inherits Animal’s constructor and speak method and adds bark. A Dog instance can call both methods. instanceof Animal is true because Animal.prototype is in the prototype chain.

Check each step: identify the input values, follow the JavaScript operation in execution order, and match the final expression with the exact output shown. An annotation can reject invalid source before execution, but does not change the value printed.

Inheritance does not copy every base method into each instance; methods are normally found through the prototype chain.

How to debug: reproduce the issue in a minimal example. For compiler errors, inspect the inferred and declared types; for a runtime surprise, log the actual values and follow their references or branches.

Add a Cat derived from Animal and give it its own meow method.

Practice routine: predict the result on paper, edit one part of the code, compile under --strict, run the JavaScript, and explain whether TypeScript caught an error or JavaScript simply behaved differently.

Class Inheritance: Inheritance lets a derived class reuse and extend a base class with the extends keyword.

Dog inherits Animal’s constructor and speak method and adds bark. A Dog instance can call both methods. instanceof Animal is true because Animal.prototype is in the prototype chain.

Before continuing, explain one valid use case, one common pitfall, and the separation between compile-time checking and runtime behavior for this feature.